L-amino acids, especially L-lysine, are used in the feeding of animals, in human medicine and in the pharmaceutical industry. They are typically produced by fermenting strains of coryneform bacteria, especially Corynebacterium glutamicum. Because of the great importance of amino acids, work is continually being done to improve production processes. Improvements may concern measures relating to the fermentation process (e.g., relating to stirring and oxygen supply) or the composition of the nutrient medium, (e.g., relating to the sugar concentration during the fermentation). They may also concern the purification of product (e.g., by ion-exchange chromatography) or the intrinsic performance properties of the microorganism itself.
To improve the performance properties of amino acid-producing microorganisms, methods of mutagenesis, selection and mutant selection are often employed. These methods may be used to obtain strains that are resistant to antimetabolites, such as, for example, the lysine analogue S-(2-aminoethyl)-cysteine, or which are auxotrophic for amino acids which are important in terms of regulation, and produce L-amino acids. In addition, methods of recombinant DNA technology have been used to improve the L-amino-acid-producing strains of Corynebacterium glutamicum by amplifying individual genes of amino acid biosynthesis. General articles on this subject include Kinoshita (“Glutamic Acid Bacteria,” in: Biology of Industrial Microorganisms, Demain and Solomon (eds.), Benjamin Cummings, London, UK, 1985, 115–142; Hilliger, BioTec 2:40–44 (1991); Eggeling, Amino Acids 6:261–272 (1994); Jetten, et al., Crit. Rev. Biotech.15:73–103 (1995); and Sahm, et al., Ann. New York Acad. Sci. 782:25–39 (1996)).